The present disclosure relates generally to uninterruptible power supplies (UPSs), and particularly to control systems for the control thereof.
UPSs are employed in a variety of applications where a constant source of power is desired at a load. A typical UPS system involves an inverter feed path, also generally referred to as the inverter, that is operably connectable in parallel with a bypass feed path, also generally referred to as the mains. The mains may be connected to a utility, but may also receive power from some other supply not connected to a utility electrical grid. The inverter may receive power from the same source as the mains, but may also receive power from some other supply.
There are basically two types of UPSs depending on their operation mode. Double conversion UPSs that offer the maximal protection level as the load is almost always fed by the inverter feed path, and line-interactive UPSs where the load is fed by the bypass feed path and the inverter is used to correct the shape of the load voltage. The double conversion type of UPS may have limited efficiency that results in higher operation costs, particularly for large units. The line-interactive type of UPS may have a better efficiency, but may not be able to maintain the quality of the output voltage when the mains frequency is variable.
The control and management of a double conversion UPS tends to keep the load on the inverter in order to maximize the protection level for the load. In some limited cases, the control has to switch the load to bypass, but as soon as possible the load is switched back on the inverter again. During these commutations, the inverter and the mains are paralleled for a short period of time, which introduces some drawbacks.
Accordingly, there is a need in the art for a control strategy allowing for the control of the paralleling between mains and inverter of a UPS system.